Sub vs. three way

No not at all. I think everyone would love a pair of speakers that go all the down to 20hz but those speaker are very very expensive. Adding a sub to monitors will give you fuller sound but bass at 20 to 80hz will be very directional. My monitors go to 70 hz (50hz at -6db) and adding a rel b3 made a world of difference (all for the better for me). Now I would like to add a second sub but most likely never will. On the other hand if you can center your sub inbetween your monitors you wont really notice the bass being as directional.

BTW, bass from a subwoofer is no more directional than from full range speakers, i.e., it is not.

Sorry Bob I have to strongly disagree. My sub is approx 1 ft to the left of my left monitor and the deep bass is louder from the left side of the room. To me that is what I am calling directional. I am also sitting in the 'sweet spot' when I hear this. I had the sub on the right of my right speaker and the deep bass was louder from the right. My room dimensions are approx the same but I'm doing a 7ft equilateral triangle.
I wish I could center the sub but I cant with my setup